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Relative growth rate (RGR) is a standardized measure of growth that accounts for the difference in initial organ size. Relative growth rate sets the sink strength potential that, in combination with dark respiration (Rd), determines the carbon need of organs. Total Rd is the sum of maintenance respiration and growth respiration (Rg). The first provides energy for the maintenance of the existing cell structures, while the latter provides energy for growth. Dark respiration is mainly driven by temperature, but it varies during the season according to temperature acclimation and organ growth. Temperature acclimation is defined as the variation of Rd following the exposure to short or long periods of different temperatures. Temperature strongly affects growth and drives the Rg component of Rd. We hypothesized that RGR has a fundamental role in Rd variation during the season. The aims of the study were to determine the following: (i) if there was a variation of leaf Rd over the season and if such variation could be due to acclimation and/or RGR; (ii) the type of acclimation (i.e., Type I or II) on fully expanded leaves and newly formed leaves; and finally, (iii) if acclimation or RGR should be included to model Rd variation over the season. Leaf Rd was measured in field-grown plants from bud break to summer. Different cohorts of leaves were used to test the effect of exposure to different temperature regimes during leaf formation. The only case of acclimation was found in fully expanded leaves. It was an acclimation of Type II. Under field conditions, acclimation of filbert leaves, Rd to temperature was limited since most of the Rd variation during the season was explained by RGR. Our work suggests that RGR is a fundamental parameter that should be included in addition to temperature to model seasonal Rd pattern., (© The Author(s) 2022. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permission@oup.com.)

Functional structural plant models of tree crops are useful tools that were introduced more than two decades ago. They can represent the growth and development of a plant through the in silico simulation of the 3D architecture in connection with physiological processes. In tree crops, physiological processes such as photosynthesis, carbon allocation and growth are usually integrated into these models, although other functions such as water and nutrient uptake are often disregarded. The implementation of the 3D architecture involves different techniques such as L-system frameworks, pipe model concepts and Markovian models to simulate branching processes, bud fates and elongation of stems based on the production of metamers. The simulation of root architecture is still a challenge for researchers due to a limited amount of information and experimental issues in dealing with roots, because root development is not based on the production of metamers. This review aims to focus on functional-structural models of fruit tree crops, highlighting their physiological components. The potential and limits of these tools are reviewed to point out the topics that still need more attention., (© The Author(s) 2021. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Xylem hydraulic conductance varies in response to changes in sap solute content, and in particular of potassium (K(+)) ion concentration. This phenomenon, known as the 'ionic effect', is enhanced in embolized stems, where it can compensate for cavitation-induced loss of hydraulic conductance. Previous studies have shown that in well-watered laurel plants (Laurus nobilis L.), potassium concentration of the xylem sap and plant hydraulic conductance increased 24 h after fertilization with KCl. The aim of this work was to test whether water-stressed laurel plants, grown under low potassium availability, could recover earlier from stress when irrigated with a KCl solution instead of potassium-free water. Two-year-old potted laurel seedlings were subjected to water stress by suspending irrigation until leaf conductance to water vapour (g(L)) dropped to ∼30% of its initial value and leaf water potential (ψ(L)) reached the turgor loss point (ψ(TLP)). Plants were then irrigated either with water or with 25 mM KCl and monitored for water status, gas exchange and plant hydraulics recovery at 3, 6 and 24 h after irrigation. No significant differences were found between the two experimental groups in terms of ψ(L), g(L), plant transpiration, plant hydraulic conductance or leaf-specific shoot hydraulic conductivity. Analysis of xylem sap potassium concentration showed that there were no significant differences between treatments, and potassium levels were similar to those of potassium-starved but well-watered plants. In conclusion, potassium uptake from the soil solution and/or potassium release to the xylem appeared to be impaired in water-stressed plants, at least up to 24 h after relief from water stress, so that fertilization after the onset of stress did not result in any short-term advantage for recovery from drought., (© The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

This study reports experimental evidence on the effect of short-term potassium fertilization on potassium uptake, tissue concentration and hydraulic conductance of pot-grown laurel plants. Potassium uptake and loading into the xylem of laurel seedlings increased within 24 h after fertilization. Potassium was not accumulated in roots and leaves, but the [K(+)] of xylem sap was 80% higher in fertilized plants (+K) than in potassium-starved plants (-K), as a likely result of recirculation between xylem and phloem. Increased xylem sap [K(+)] resulted in a 45% increase in transpiration rate, a 30% increase in plant hydraulic conductance (K(plant)) and a 120% increase in leaf-specific conductivity of the shoot (k(shoot)). We suggest that this increase was due to ion-mediated up-regulation of xylem hydraulics, possibly caused by the interaction of potassium ions with the pectic matrix of intervessel pits. The enhancement of hydraulic conductance following short-term potassium fertilization is a phenomenon that can be of advantage to plants for maintaining cell turgor, stomatal aperture and gas exchange rates under moderate drought stress. Our data provide additional support for the important role of potassium nutrition in agriculture and forestry.